1. Field of the Invention
The present invention relates to a paper feeding device with a constitution in which an envelope or the like with a non-uniform thickness across its whole surface, that is, a recording media having deviations in thickness is stably fed.
2. Description of the Related Art
An image forming apparatus of a copier, a printer, a facsimile, a hybrid machine thereof or an ink jet printer or the like includes a constitution in which a bulk of a recording media stacked on a paper feeding tray is fed piece by piece. Because the image forming apparatus is used for purposes of mechanistic diversity, the image forming apparatus is required to be adapted to various recording medias of differing size, thickness and material or the like. For example, an envelope is formed to have a bag shape. The envelope is obtained by bending over and sticking a piece or a plurality of pieces of sheet parts. Therefore, a thickness of an envelope is non-uniform. As a result, such envelopes in stacked form becomes difficult to be stably fed by a paper feeding device having a mechanism only suited for carrying a sheet shaped recording media of an uniform thickness. That is, when a paper feeding device designed for feeding a recording media of an uniform thickness is used to feed stacked envelopes, due to partial differences in thickness, there are cases in which feeding failure, oblique feeding, dog ears and paper jamming or the like are generated. Devices to solve such defects are respectively proposed by the following patent documents. In Japanese utility model registration No. 2560481, a constitution is disclosed in which a paper feeding axis is profiled to be parallel to a tilt of a sheet surface. In JP2002-284376A, a constitution is disclosed in which feeding is performed by a plurality of rollers and a transmission device of the drive forces of the rollers is disposed between the plurality of rollers. In JP2004-269070A, a constitution is disclosed in which an attachment is disposed on a bottom plate. The attachment is projected and has a plane shaped upper surface. However, recently, envelopes of a variety of sizes and structure are required to be handled by a paper feeding device of an image forming apparatus. Therefore, a variety of propositions have been made but no paper feeding device has been proposed heretofore in which stable feeding is successful with regard to an angle bottomed envelope with a large capacity and with cargo spaces formed by folding in both side surfaces and a bottom surface. That is, cargo spaces formed by valley folding are present in a bottom part and both side surfaces of an envelope so that a large capacity can be obtained without enlarging an external size of when the envelope is developed. In recent years, an envelope with such a merit is particularly preferable in businesses that handle envelopes with a large capacity, for example, in prescription works that handle pharmaceutical bags. In envelopes with cargo spaces, there is an envelope in which cargo spaces are formed in both side surfaces of the envelope. There is also a so called angle bottomed envelope in which cargo spaces are formed in both side surfaces and a bottom surface.
FIGS. 9A and 9B are a perspective view and a front elevational view that illustrates a structure of an angle bottomed envelope. The angle bottomed envelope 100 has a structure in which a bottom cargo space part 102 is folded back at a folding over part 101. With such a structure, when the angle bottomed envelope is in a state folded back, a thickness of the angle bottomed envelope becomes non-uniform. Therefore, when such envelopes are stacked in a bulk form as a recording media, differing parts has differing layer thicknesses. The number within FIG. 9B illustrates an example of the number of sheet pieces layered in each part of an envelope. In this example, twelve pieces of sheets are layered in a thickest part. Two pieces of sheets are layered in a thinnest part. As just described, the thickest part has a thickness six times to that of the thinnest part. When such an angle bottomed envelope is fed as a recording media through a paper feeding device of an image forming apparatus, conventional technologies illustrated in the above described patent documents cannot obtain a sufficient feeding quality. Next, FIGS. 10A and 10B illustrates a general paper feeding device equipped in an image forming apparatus. The paper feeding device is approximately constituted from a paper feeding tray 111 and a paper feeding roller (feeding device) 115. The paper feeding tray 111 includes a constitution in which a bottom plate 113 is supported to be freely movable upwardly and downwardly inside a casing 112. The bottom plate 113 is a device that sends up a recording media stacked thereon to a paper feeding position. The paper feeding roller is a device that feeds piece by piece towards an image forming part an uppermost piece of the recording media sent up to the paper feeding position. FIG. 10B illustrates a state in which a recording media P is stacked on the bottom plate 113. By a drive device disposed externally, the bottom plate rotates upwardly with one side thereof as a center of rotation to send up the recording media. FIG. 11A illustrates a state in which the angle bottomed envelopes as a recording media are stacked in a bulk. FIG. 11B is a front elevational view during descending of the bottom plate. FIG. 11C is a front elevational view during ascending of the bottom plate. As illustrated in FIG. 11A, when the angle bottomed envelopes are stacked, a state is present in which non-uniformity in thickness is accumulated. The bottom plate 113 rotates around a rotating center 113a and sends upwardly the angle bottomed envelope 100. When the angle bottom envelope is sent up to a certain height, the rotation stops. For a method that detects the height at this moment, a detection device can be disposed separately or a mechanism that detects a height of a recording media can be disposed in a carrying device. FIG. 11D further illustrates in detail from a front surface side the state at this moment. As illustrated in FIG. 11D, a density around a center of a width direction of the angle bottomed envelopes 100 is comparatively low to a peripheral part because a number of sheet pieces layered around the center is much less than that of the peripheral part. Therefore, when a central part of the angle bottomed envelope contacts the feeding roller 115, if the bottom plate 113 is further rotated upwardly, a state is present in which the feeding roller gradually sinks into the central part of the stack of the angle bottomed envelope 100. When the feeding roller 115 is rotated in this state by a second drive device disposed externally, a phenomenon of feeding failure is generated as a result because frictional forces between the feeding roller 115 and the angle bottomed envelope 100 become insufficient. In addition, oblique feedings occur due to non-uniform frictional forces generated. In addition, dog ears and paper jamming are also generated because a vicinity of both side surfaces of the angle bottomed envelope 100 in a vicinity of a tip edge of the angle bottomed envelope 100 in the feeding direction contact an external member.